1. Field of the Invention
The invention relates to a measurement transducer and, more particularly, to a measurement transducer having a high level of accuracy across a wide temperature range.
2. Description of the Related Art
Field devices for process instrumentation that are used to record and/or influence process variables, for example, and are connected to one another via an automation network, i.e., with a field bus, for the purpose of interchanging data are often used in automation technology. Field devices that record a physical or chemical variable as a process variable are often referred to as measurement transducers because they convert the respective variable into a measured value and output this value to a superordinate control station, for example, or, as an actual value, to a regulator for further processing. Examples that may be mentioned are measurement transducers for temperature, pressure, flow rate, flow velocity, filling level, density, pH, conductivity or gas concentration of a medium.
DE 10 2006 024 742 A1 discloses a modular measurement transducer for magnetoinductive flow measurement. This transducer comprises a pick-up for recording the flow velocity of a medium flowing through a measuring tube and for generating a corresponding measurement signal. Analog components for recording the physical variable and for converting this variable into an electrical signal which can be processed further, as well as electronic components for generating a digital measurement signal, are combined in a pick-up housing. Signal preprocessing therefore occurs in an electronic unit close to the sensor and there is no need to transmit analog signals over relatively long connecting lines. The sensitivity of the measurement transducer to external interference, which could distort analog signals, is therefore comparatively low and good measurement accuracy is achieved. An evaluation device that is accommodated in a housing separate from the pick-up housing is used to further evaluate the measurement signal, such as to linearize the measurement signal, so as to perform monitoring and diagnostic functions and to transmit measured values. Based on the measurement signal, the evaluation device therefore calculates a measured value for the flow velocity and outputs the measured value to a superordinate control station or a flow rate regulator, for example, via the communication interface for further processing. The pick-up and the evaluation unit are often also referred to as a sensor and a transmitter, in which case, in a manner deviating therefrom, the term “sensor” in connection with this application is intended to be understood as meaning only the component that is used to convert the physical or chemical variable into an electrical signal which can be processed further.
Process instrumentation field devices are often approved for use in an ambient temperature range of −40 to +85° C., this including the complete device with the pick-up and the evaluation device. In addition, there are measurement transducers in which only the pick-up is approved for an extended temperature range. For example, pick-ups for pressure measurement transducers, which have a pressure measurement cell filled with oil as the pressure transfer medium, are often approved for a range of use of up to +100° C. This limit is justified, on the one hand, by the quality of the silicone oil in which undesirable bubbles may be produced by degassing previously dissolved gases at relatively high temperatures and, on the other hand, by the sensor used and its electronics close to the sensor.
However, in certain applications, the temperature of the process medium is so high that mounting of a pressure sensor, for example, in the direct vicinity of the process medium would destroy the sensor or would at least reduce the measurement accuracy. In order to nevertheless enable a measurement, DE 10 2004 060 647 B4, for example, discloses the use of a pressure transmitter between the process connection and the sensor. A long capillary tube that is filled with a high-quality fluid as the pressure transfer medium and spatially separates the pressure measuring point with the hot process medium from the pick-up is situated between the process connection, which is referred to as a remote process sealing unit there, and the sensor. The pressure is therefore transferred over a relatively long distance using the incompressible fluid. However, the disadvantage is still possible inaccuracy on account of undesirable bubble formation in the fluid and certain distortion of the measured pressure value that may be produced on account of the remaining, variable compressibility caused by sorbed gas inclusions, for example, mass inertia and the viscosity of the filling fluid. In addition, the use of such a pressure transmitter is associated with a considerable amount of additional effort.